The invention relates generally to a process for reforming hydrocarbons in a reforming unit, and more particularly, to an improved process for reforming methane and light hydrocarbons with an integrated autothermal reformer and cogeneration power plant, resulting in the production of synthesis gas, synthesis gas by-products, and power with an improved thermal efficiency.
Processes for reforming of light hydrocarbons to produce various synthesis gases and synthesis gas products are well known in the art. Conventional processes for reforming of light hydrocarbons use steam or oxygen in a reformer.
Steam reforming of light hydrocarbons, to produce hydrogen and carbon monoxide, as shown in (1), is a widely used commercial process. EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3 H.sub.2 (1)
.DELTA.H=49.3 KCAL/mol PA1 .quadrature.H=-8.5 KCAL/mol
Due to the presence of excess steam, some of the carbon monoxide and steam react simultaneously as indicated by the water gas shift reaction (2). EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (2)
In the refining industry, steam reforming is a major component of most hydrogen production complexes. Approximately 90% of the hydrogen from a hydrogen plant is produced directly by steam reforming in a steam reformer reactor. The remaining 10% is produced via the water-gas shift process which requires CO produced in the reformer. Steam reforming is also an integral component in methanol production from natural gas (3) as well as in Fischer-Tropsch processes (4) EQU CO+2H.sub.2 .fwdarw.CH.sub.3 OH (3) EQU CO+2H.sub.2 .fwdarw.1/n(CH.sub.2).sub.n +H.sub.2 O (4)
In response to increasing environmental concerns, the demand for hydrogen and methanol are expected to increase, leading to a need for additional reforming capacity. Therefore, improved integrated processes, resulting in improved efficiency and, hence, lower utility costs, are timely and attractive options.
Steam reforming is traditionally carried out in multitubular fixed bed reactors which are heated on the outside in a furnace. The disadvantages of traditional steam reforming in multitubular fixed bed reactors are described in U.S. Pat. No. 5,624,964.
One approach for eliminating some of the disadvantages of the multitubular fixed bed reactors for steam reforming processes, e.g., eliminating costly heat transfer surfaces, is through the use of two communicating fluid beds, either of which could be an upflow or downflow fixed bed, a fast fluid bed or a circulating fluid bed. In such a design, reforming catalyst is heated directly, via combustion of fuel gas, in one of the fluid beds in a combustor-regenerator, and then the hot catalyst is conveyed to the other fluid bed in a reformer reactor, in which the steam reforming reaction is carried out. In this way, the heat gained in the bed in which the combustion is carried out can be transferred directly to the reformer section supplying the required sensible heat and endothermic reaction heat for the reforming reaction (1). The recycling of the reforming catalyst to a combustion zone also regenerates the catalyst by burning off any coke formed during the reforming reaction. Since the continuous regeneration eliminates concerns over continuous coke build-up and, hence, permanent catalyst deactivation, lower steam to carbon ratios can be used resulting in further utility savings.
One major obstacle to such a design is the fact that steam reforming is normally carried out under pressure (150-400 psig) and, therefore, the air required for combustion and, hence, catalyst heating, must be compressed to preserve the pressure balance in the catalyst circulation loop. The cost of energy required for this compression is very high and, to some degree, off-sets the improved heat transfer benefits relative to the traditional non-contact heat transfer. A portion of the energy expended to compress the external gases sent to the combustor-regenerator can potentially be recovered by expanding the hot, pressurized gases exiting the combustor-regenerator, after separation from the solids, to a turbine to produce power. The inability of conventional turbines to operate at high temperatures (&gt;1400.degree. F.) with entrained particulates, due to excessive turbine blade erosion, provides a second major process obstacle. Filtration of hot gases is an option only if the flue gas is first cooled to a temperature for which commercial catalyst filters are available. Cooling via some external medium entails a further reduction in the thermal efficiency. At these low temperatures very little net energy is gained in excess of the energy required for compression which leads to a high investment cost and a loss in thermal efficiency.
These obstacles were overcome in U.S. Pat. No. 5,624,964, by integration of a steam reforming process, composed of two communicating fluid beds, with a cogeneration power plant wherein a fluidized bed process unit is integrated with a combined cycle power plant. In U.S. Pat. No. 5,624,964 the integration, in part, involves drawing off a portion of compressed air from a compressed air flow from a gas turbine power plant compressor and this "borrowed" compressed air is introduced into a combustor-regenerator of a steam reforming unit together with fuel gas. In U.S. Pat. No. 5,624,964, a small booster compressor for the borrowed air may be used to compensate for pressure drop in the combustor, and the "borrowed" compressed air and also some extra heat is later returned to the power plant by mixing hot, compressed off gases from the combustor-regenerator with the power plant compressed air flow which is being conveyed to the power plant combustor. The mixing of the hot flue gases and the rest of the compressed air flow lowers the temperature of the off gases sufficiently to allow removal of catalyst fines by filtration, without any thermodynamic losses. At the same time, the temperature and pressure of the air flow to the power plant combustor in the integrated process of U.S. Pat. No. 5,624,964 are increased to facilitate combustion.
Although the integrated process of U.S. Pat. No. 5,624,964 has many advantages, such as, e.g., a) increased efficiency by elimination of the need for a large air compressor for the combustor-regenerator section of the steam reforming process, b) thermally efficient utilization of the energy of hot combustion gases by mixing with the excess cold air to permit filtration of the particulates with no thermodynamic efficiency loss compared to non-integrated gas turbines, c) reduction of the need to maintain very low single pass coke production since catalyst is continuously regenerated, allowing a reduction in excess steam to the reformer, and d) reduction in combined cycle power plant compression and combustion costs through the introduction of hot, compressed gases from the fluidized bed regenerator, it is always desirable to increase efficiency and reduce the cost of plants and processes which thermally reform hydrocarbons.
Oxygen reforming of light hydrocarbons, to produce hydrogen and carbon monoxide, as shown in the net reaction (5) is well known. EQU CH.sub.4 +O.sub.2 .fwdarw.CO+H.sub.2 +H.sub.2 O (5)
Water in the form of steam is generated in this process and reacts with carbon monoxide as shown in (2) above, and oxygen reforming can also be an integral component in methanol production as shown in (3) above and in the Fischer-Tropsch process as shown in (4) above.
Traditionally, the autothermal reforming reaction with oxygen has been carried out by co-feeding pure oxygen to the reactor along with methane or other hydrocarbons along with steam. Unfortunately, the use of pure oxygen requires the use of capital intensive cryogenic units for air separation. In fact, it has been estimated that 50% of the cost of an autothermal reformer is associated with the expense of air separation. Alternatively, if air is used as the source of oxygen in traditional autothermal reforming, it introduces nitrogen diluent to the syngas. This is a significant disadvantage because nitrogen diluents effect the synthesis gas products and by-products, increase the size of plant equipment, adversely effect heat duties and significantly reduce separation efficiency of the synthesis gas products and by-products.
The concept of utilizing a metal oxide to react with methane to yield syngas is disclosed by Lewis et al. in Industrial and Engineering Chemistry, Vol. 41, No. 6, 1227-1237 (1949). In this study, Lewis et al. investigated the use of copper oxide to aid in the autothermal reforming of methane and fed the solid powder from a reservoir tube into the gas stream which carried the powder into the reactor. An example of the overall reaction for reacting a metal oxide with methane to produce syngas is shown in (6) below: EQU CH.sub.4 +2MO.sub.x+1 .fwdarw.H.sub.2 +CO+2MO.sub.x +H.sub.2 O(6)
where x is an integer which renders the metal oxide charge neutral.
Although Lewis et al. indicate that energy is released in two stages (1) oxidation of the hydrocarbon by the metal oxide and (2) reoxidation of the metal oxide and that the metal oxide can be reoxidized by air, the process and apparatus of Lewis et al. are disadvantageous because they underutilize the process and do not make use of the economic value of the heat, including the hot, compressed off gas in a combustor-regenerator.